Antifreeze concentrates and coolant compositions containing these concentrates for cooling circuits in internal combustion engines

ABSTRACT

A description is given of antifreeze concentrates which comprise 
     a) at least one water-miscible alcohol which lowers the freezing point; 
     b) at least one corrosion inhibitor; and 
     c) as reserve alkalinity donor, at least one mono- or polycyclic aromatic compound which has at least one hydroxyl group and a high buffer capacity in the pH range from about 6 to 10 
     and of coolant compositions which comprise said concentrates and are intended for the cooling circuits in internal combustion engines.

The present invention relates to novel antifreeze concentrates based onalkylene glycols and to ready-to-use aqueous coolant compositions whichcomprise said concentrates and are intended for cooling circuits ininternal combustion engines.

Coolants for the cooling circuits of internal combustion engines, suchas in motor vehicles, generally include alkylene glycols, especiallyethylene glycol or propylene glycol, or else corresponding glycolethers. For use in the cooling system they are diluted with water andare intended not only to provide frost protection by lowering thefreezing point of water but also to contribute toward good heatdissipation. Alkylene glycol/water mixtures, however, are extremelycorrosive at the operating temperatures of internal combustion enginesand so the various metals used to construct cooling systems must beprotected against corrosion.

As far as thermal loading of the heat transfer areas, pressure, flowrate, and the selection of materials are concerned, the operatingconditions in modern internal combustion engines nowadays place muchgreater demands on the corrosion prevention capacity of the coolant thanwas hitherto the case. In addition to the known materials such ascopper, brass, soft solder, steel, and grey cast iron, there is alsoincreasing use of alloys of aluminum for the purpose, in particular, ofweight reduction. Consequently, there is increased occurrence in morerecent literature of descriptions of specific combinations of long-knownactive substances, each claimed to have a specific spectrum of action.

In particular, alkali metal silicates have been found excellent forprotecting aluminum against corrosion. In glycol-based radiatorprotectants, however, they tend to form gels or even to precipitate, socreating a need for special silicate stabilizers.

Examples of constituents of such combinations of active substancesinclude salts of organic acids which, especially in the form of theiralkali metal salts, are efficient corrosion inhibitors, whereas the freeacids are less effective or may even be corrosive. The incidence of freeacids in radiator protectant formulations as a possible result, forexample, of oxidation processes or the ingress of nitrous gases into thecooling system is therefore highly undesirable. It is for this reasonthat radiator protectant formulations contain small amounts of agentsknown as reserve alkalinity donors which have a buffer action in therelevant pH range and so prevent the formation of free acids. Examplesof compounds employed as reserve alkalinity donors are amines andphosphates and, in particular, the less expensive borax. Such corrosioninhibitor compositions are described, for example, in EP-B-0 229 440 andEP-A-0 308 037.

Because of the above-described disadvantages associated with the use ofsilicates as corrosion inhibitors for aluminum, in recent developments atrend has been noted to omit them and instead use other activesubstances. Under these conditions, however, the reserve alkalinitydonor, borax, is highly corrosive, and so can no longer be used. The useof phosphates in modern-day radiator protectant formulations is likewiseundesirable. This applies to an even greater extent to amines, on thebasis of their characterization as potential nitrosamine formers.

WO-A-96/18757 describes anticorrosion compositions for temporarilyprotecting metallic surfaces, especially those comprising iron, againstcorrosion. Said compositions are aqueous solutions comprising acombination of a carboxylic acid anion of 6 to 44 carbon atoms with anaromatic hydroxy compound having a pK_(a) for the hydroxyl group in therange from 7.0 to 11, in a weight ratio of from 1:2 to 20:1. Corrosioninhibitor systems of that kind, however, are not intended for use inalkylene glycol-containing coolant systems for internal combustionengines. Instead, it is proposed that they be used in aqueous treatmentbaths in which the articles to be treated—for example, steel sheets—aredipped.

Consequently, there continues to be a need for improved antifreezeconcentrates.

It is an object of the present invention to provide silicate-freeantifreeze concentrates which comprise new reserve alkalinity donors andare unhampered by the disadvantages described above. The antifreezeconcentrates should in particular exhibit enhanced corrosion inhibitionon aluminum or its alloys.

We have found that this object is achieved by taking conventionalsilicate-free inhibitor systems for alkylene glycol-containing radiatorprotectant formulations and replacing the customary reserve alkalinitydonors, such as borates, phosphates and amines, by a mono- or polycyclicaromatic compound which has at least one hydroxyl group and a highbuffer capacity in the pH range from about 6 to 10.

The present invention therefore provides, in particular, antifreezeconcentrates which comprise

a) at least one water-miscible alcohol which lowers the freezing point,preferably selected from alkylene glycols and alkylene glycol ethers;

b) at least one corrosion inhibitor; and

c) as reserve alkalinity donor, at least one mono- or polycyclicaromatic compound which has at least one hydroxyl group and a highbuffer capacity in the pH range from about 6 to 10.

Such antifreeze concentrates and the ready-to-use coolant compositionsprepared with them have the surprising advantage over conventionalformulations of a markedly better corrosion-inhibiting effect. Inparticular, an improvement is observed in the corrosion stability ofaluminum materials, as used in engine construction.

In one preferred embodiment the antifreeze concentrates of the inventioncomprise as reserve alkalinity donor at least one compound of theformula I

Ar—X  (I)

where

Ar is a radical of the formula II

where

n is an integer from 0 to 4 and

R¹ is hydrogen or an alkali metal ion, and the substituents

R² independently of one another are a hydroxyl, alkyl, hydroxyalkyl,heteroalkyl or hydroxyheteroalkyl group, and

X is —COOH, —SO₃H or —SO₂Ar, where Ar is as defined above.

Preferred compounds of the formula I include Ar radicals of the formulaII where n is 0, 1 or 2. The ring substituents R², where present, arepreferably and independently of one another hydroxyl or hydroxyalkylgroups. The radical X is preferably —SO₂Ar, where Ar can be as definedabove.

A reserve alkalinity donor which is particular preferred in accordancewith the invention is selected from compounds of the formula III

where R¹, R² and n independently of one another are as defined above. Incompounds of formula III R¹ is preferably hydrogen, n is preferably 0, 1or 2 and R², if present, is preferably hydroxy or hydroxyalkyl.Compounds of the formula III are preferably symmetrical; that is, thetwo aromatic rings are substituted mirror-symmetrically starting fromthe central SO₂-group.

Particular preference is given to compounds of the formula III where theradicals R¹ are identical and are hydrogen or an alkali metal and n ineach case is 0. Specific examples of such compounds are4,4′-dihydroxydiphenyl sulfone (bisphenol S) and the correspondingpositional isomers, and also mixtures of these compounds.

Where R¹ in the abovementioned compounds of the formula I is nothydrogen, it is a preferably monovalent metal cation, especially analkali metal cation such as, for example, a sodium or potassium cation.Where two or more radicals R¹ are present, they are preferably the samealkali metal cation.

Alkyl radicals suitable in accordance with the invention arestraight-chain or branched carbon chains having 1 to 10, preferably 1 to6 and, in particular 1 to 4 carbon atoms. Examples that may be mentionedinclude the following radicals: methyl, ethyl, n-propyl, i-propyl,n-butyl, sec-butyl, i-butyl, t-butyl, n-pentyl, sec-pentyl, isopentyl,n-hexyl, 1-, 2- or 3-methyl-pentyl, and long chain alkyl radicals, suchas unbranched heptyl, octyl, nonyl and decyl, as well as the singly ormultiply branched analogs thereof. Hydroxyalkyl radicals suitable inaccordance with the invention are preferably hydroxyl-containing alkylradicals as defined above. They may contain 1 to 10, preferably 1 to 3and, in particular 1 hydroxyl group, and at least one hydroxyl group ispreferably in terminal position, i.e., attached to a primary carbonatom.

Heteroalkyl radicals which can be used in accordance with the inventionare alkyl groups as defined above in which at least one heteroatom,selected S, N and O, preferably O, is present in the alkyl chain.Examples of particularly preferred heteroalkyl groups are methoxymethyland ethoxyethyl groups.

Hydroxyheteroalkyl groups which can be used in accordance with theinvention are heteroalkyl groups as defined above in which at least 1 to10, such as 1 to 3, hydroxyl groups, especially 1 hydroxyl group, are/isattached to the heteroalkyl chain. Possible examples of such radicalsare hydroxyethyloxyethyl and hydroxymethyloxymethyl.

Abovementioned alkyl, hydroxyalkyl, heteroalkyl and hydroxyheteroalkylgroups in accordance with the invention may also, if desired, beattached to the aromatic ring by way of a heteroatom, preferably anoxygen atom.

In another preferred embodiment of the invention the antifreezeconcentrates comprise as corrosion inhibitor at least one compoundselected from triazoles, thiazoles, and mono- or polybasic, aliphatic oraromatic carboxylic acids or carboxylic salts, and mixtures of thesecompounds.

Examples of thiazoles and triazoles which can be used in accordance withthe invention are derivatives of these compounds having a condensed,preferably aromatic ring system. Particular mention may be made ofbenzotriazole, benzothiazole, toluthiazole and tolutriazole.

The abovementioned thiazoles and triazoles may be present individuallyor in a mixture in the antifreeze concentrates of the invention. Theymay also be present as a mixture with at least one carboxylic acid ofthe type specified above.

Examples of carboxylic acids suitable in accordance with the inventionare straight-chain or branched aliphatic monocarboxylic acids of 5 to 12carbon atoms, examples being pentanoic, hexanoic, octanoic, nonanoic,decanoic, undecanoic, dodecanoic, 2-ethylhexanoic and isononanoic acid.

Aromatic monocarboxylic acids suitable in accordance with the inventionare, in particular, those of 7 to 16 carbon atoms. Possible examples arebenzoic acid, alkylbenzoic acids where the alkyl moiety is as definedabove, such as o-, m- and p-methylbenzoic acid, and hydroxyl-containinganalogs of the above aromatic monocarboxylic acids, such as o-, m- orp-hydroxybenzoic acid and o-, m- or p-(hydroxymethyl)benzoic acid.

The abovementioned carboxylic acids which can be employed in accordancewith the invention may if desired be substituted additionally by groupscontaining ether oxygen, such as, for example, alkoxyalkyl groups, or bycarbonyl-functional groups, such as alkylcarbonyl groups, the alkylmoiety in each case being as defined above.

The carboxylic acids utilizable in accordance with the invention can beadded individually or as a mixture to the antifreeze concentrates of theinvention. Particular preference is given to mixtures of at least twocarboxylic acids of the abovementioned type. Two carboxylic acids arepresent preferably in a molar ratio of from about 1:99 to 99:1, inparticular from 3:97 to 97:1. A particularly preferred mixture is thatof two dicarboxylic acids, especially two aliphatic dicarboxylic acids,and/or their salts, especially alkali metal salts, such as sodium orpotassium salts, for example.

Further carboxylic acid mixtures utilizable in accordance with theinvention comprise a mixture of two monocarboxylic acids, especially ofone aliphatic and one aromatic monocarboxylic acid, and mixtures of amonocarboxylic with a dicarboxylic acid, especially an aliphaticdicarboxylic acid with a branched aliphatic monocarboxylic acid.

As a specific example of a carboxylic acid mixture that can be used inaccordance with the invention mention may be made of a mixture ofsebacic and adipic acids.

Mixtures suitable in accordance with the invention and comprisingcarboxylic acids and triazole and/or thiazole compounds are described inthe earlier EP-A-0 816 467, the content of which is expresslyincorporated herein by reference. Suitable mixtures comprise, forexample, at least one carboxylic acid of the type designated above andat least one thiazole or triazole of the type designated above, in aquantitative ratio of from about 1:20 to about 20:1, in particular fromabout 1:15 to about 15:1.

The antifreeze concentrates of the invention commonly include thereserve alkalinity donor(s) of the above formula I in a proportion offrom about 0.05 to 4% by weight, preferably from about 0.5 to 3% byweight and, in particular, from about 0.8 to 2% by weight.

The abovementioned corrosion inhibitors, i.e., carboxylic acids,thiazoles and/or triazoles, are commonly present in an overallproportion of from about 0.1 to 20% by weight, in particular from 0.5 to10% by weight, based on the overall weight of the concentrate. In thiscase the carboxylic acids of the type designated above may be present ina proportion of from about 0.05 to 10% by weight, preferably from 0.1 to5% by weight, based on the overall weight of the concentrate. Triazolesand/or thiazoles are commonly present in a proportion of from 0.01 to 3%by weight, preferably from 0.05 to 1% by weight, based on the overallweight of the concentrate.

As additional corrosion inhibitors, the antifreeze concentrates of theinvention may include up to 2% by weight, in particular from 0.001 to 1%by weight, based on the overall weight of the concentrate, of at leastone quaternized imidazole. Such quaternized imidazoles are described inDE-A-196 05 509. Examples are benzyl chloride-, benzyl bromide-, methylchloride-, methyl bromide-, ethyl chloride-, ethyl bromide-, diethylsulfate- and, in particular, dimethyl sulfate-quaternized1-methylimidazole, 1-ethylimidazole, 1-(β-hydroxyethyl)imidazole,1,2-dimethylimidazole, 1-phenylimidazole, benzimidazole and inparticular, N-vinylimidazole.

As further additional corrosion inhibitors, the antifreeze concentratesof the invention may include up to 2% by weight, in particular from0.001 to 1% by weight, based on the overall weight of the concentrate,of at least one soluble magnesium salt of organic acids, especiallycarboxylic acids. Examples of such magnesium salts are magnesiumbenzenesulfonate, magnesium methanesulfonate, magnesium acetate andmagnesium propionate.

In addition to the specified inhibitor components it is also possible,for example, for hydrocarbazoles to be employed, in customary amounts.

The antifreeze concentrates of the invention may additionally include upto 1% by weight, in particular from 0.01 to 0.5% by weight, based on theoverall amount of concentrate, of hard water stabilizers based onpolyacrylic acid, polymaleic acid, acrylic acid-maleic acid copolymers,polyvinylpyrrolidone, polyvinylimidazole,vinylpyrrolidone-vinylimidazole copolymers, and/or copolymers ofunsaturated carboxylic acids and olefins.

The pH of the antifreeze concentrates of the invention lies customarilywithin the range from 7 to 10, preferably from 7.5 to 9.5 and, inparticular, from 8.0 to 9.5. The desired pH is generally established byadding alkali metal hydroxide, ammonia or amines to the formulation,with solid sodium hydroxide and potassium hydroxide and also aqueoussodium and potassium hydroxide solutions being particularly suitable forthe purpose.

The aliphatic or aromatic mono- and/or dicarboxylic acids arejudiciously added directly as the corresponding alkali metal salts so asto lie automatically in the desired pH range. Alternatively, thesecarboxylic acids can be added as free acids, after which the formulationis neutralized with alkali metal hydroxide, ammonia or amines and thedesired pH range is established. Similar comments apply to the reservealkalinity donors of the formula I, which can be employed as alcohol orin the form of the corresponding alkali metal salts. Partial conversionof the compounds to their salts would be likewise conceivable.

Suitable liquid-alcoholic freezing-point reducers, which normally makeup the majority (generally at least about 80% by weight, in particularabout 90% by weight) of the antifreeze concentrates of the invention,are alkylene glycols or derivatives thereof, especially propylene glycoland, in particular, ethylene glycol. However, higher glycols and glycolethers are also suitable, examples being diethylene glycol, dipropyleneglycol, and glycol monoethers, such as the methyl, ethyl, propyl andbutyl ether of ethylene glycol, propylene glycol, diethylene glycol anddipropylene glycol. It is also possible to use mixtures of these glycolsand glycol ethers. Preference is given to ethylene glycol alone or tomixtures of alkylene glycols, i.e., ethylene glycol, propylene glycol,higher glycols and/or glycol ethers, containing at least 95% by weightof ethylene glycol.

The present invention provides, moreover, ready-to-use aqueous coolantcompositions of reduced freezing point, especially for radiatorprotection in the automotive sector, which comprise water and from 10 to90% by weight, preferably from 20 to 60% by weight, of the silicate- andborate-free antifreeze concentrates of the invention.

The present invention additionally provides for the use of compounds ofthe formula I as reserve alkalinity donors in coolant compositions basedon alkylene glycols or alkylene glycol ethers, said coolant compositionsbeing essentially free from silicate and borate.

The present invention is illustrated present by the following examples.

EXAMPLES

a) The inventive antifreeze concentrates 2, 3 and 4 and the comparativeconcentrates C1, C5, C6 and C7 described in Table 1 were prepared andtested for their corrosion protection effect. The experimental data aresummarized in attached Table 2.

TABLE 1 Composition of the antifreeze concentrates used Example C1 2 3 4C5 C6 C7 Monoethylene  93.32³⁾ 91.78 91.98 91.37 91.89 92.79 93.33glycol NaOH, 50%  2.98 3.52 3.32 3.43 3.41 2.61 — KOH, 50% — — — — — —2.97 Sebacic acid¹⁾ 2.8 2.8 2.8 2.8 2.8 1.4 0.25 Adipic acid²⁾ 0.7 0.70.7 0.7 0.7 — — 2-Ethylhexanoic acid — — — — — — 3.25 Isononanoic acid —— — — — 3.0 — Tolutriazole 0.2 0.2 0.2 0.2 0.2 0.2 0.24,4′-Dihydroxydiphenyl — 1.0 1.0 1.5 — — — sulfone Borax — — — — 1.0 — —Total 100.00  100.00 100.00 100.00 100.00 100.00 100.00¹⁾HO₂C—(CH₂)₈—CO₂H ²⁾HO₂C—(CH₂)₄—CO₂H ³⁾Parts by weight

TABLE 2 Technical data and test results Example C1 2 3 4 C5 C6 C7 pH of8.5 9.1 8.5 8.5 8.5 8.3 8.4 concentrate pH, 33% by 8.1 8.6 8.1 8.0 9.18.1 8.1 vol. Reserve alkalinity¹⁾ to pH 7.0  0.5²⁾ 6.9 3.5 5.0 11.5 0.50.3 to pH 5.5 9.5 15.5 13.0 16.5 21.0 8.0 5.2 Corrosion test³⁾ Copper ±0.00⁴⁾ +0.02 −0.04 −0.01 ±0.00 −0.01 +0.00 Soft solder −0.02 +0.01−0.05 −0.04 −0.19 −0.02 −0.07 Brass −0.10 +0.02 −0.02 −0.03 −0.01 +0.01±0.00 Steel −0.10 +0.06 −0.02 −0.03 +0.03 +0.04 ±0.00 Gray cast +0.08+0.09 −0.06 +0.01 +0.06 +0.09 +0.04 iron Cast −0.28 −0.19 −0.19 −0.19−1.01 −0.46 −0.32 aluminum Hot corrosion −26.4⁶⁾  −43.6 −23.1 −25.1−1657.7 −52.8 −42.7 test⁵⁾ AlSi₁₀Mg ¹⁾In accordance with ASTM D 1121 (of10 ml of concentrate) ²⁾Amount (ml) of 0.1 N HCl consumed ³⁾Inaccordance with ASTM D 1384 for 33% by volume solution in H₂O ⁴⁾Weightchange in mg/cm² ⁵⁾In accordance with MTU; for 20% by volume solution inH₂O ⁶Weight change in mg/sample

The results set out in Table 2 from the determination of the reservealkalinity in accordance with ASTM D 1121 clearly illustrate theeffectiveness of the dihydroxydiphenyl sulfone additive employed inaccordance with the invention. Although not quite achieving the level ofeffectiveness of borax (Example 5), the performance is outstandingrelative to the formulations (Examples 1, 6, 7) without an additive ofthe invention.

The corrosion tests in accordance with ASTM D 1384 illustrate thatadditive treatment afforded to the radiator protectant formulations, inaccordance with the invention, has no adverse effect on the corrosionbehavior and that in the case of cast aluminum, indeed, a slightimprovement can be obtained.

Table 2 also shows results obtained in the dynamic hot chamber corrosiontest of the MTU (German Engine and Turbine Union), a test with elevatedthermal loading. Here, a very high corrosion rate for theborax-containing formulation 5 is found whereas much lower values arefound for the formulations of the invention, especially in Examples 3and 4.

b) For the following series of tests an active substance concentrate inaccordance with Example 7 of WO-A-96/18757 was prepared (monoethyleneglycol 50.1 parts, 50% potassium hydroxide solution 24.9 parts, sorbicacid 20 parts and bisphenol S 5 parts) and was investigated in acorrosion test in accordance with ASTM D1384. For this test a 33% (v/v)solution of the mixture in distilled water was prepared, and 0.05 partby weight of tolutriazole was added. The results are summarized in Table3 below.

TABLE 3 ASTM D1384 - Result Weight changes Metals mg/cm² Copper −0.15Soft solder −0.83 Brass −0.31 Steel −0.01 Gray cast iron +0.03 Castaluminum −1.24

The results show that it is impossible to obtain satisfactory corrosionprotection on the basis of a composition in accordance withWO-A-96/18757.

We claim:
 1. An antifreeze concentrate which comprises a) at least onewater-miscible alcohol which lowers the freezing point; b) at least onecorrosion inhibitor; and c) as reserve alkalinity donor, at least onemono- or polycyclic aromatic compound which has at least one hydroxylgroup and a high buffer capacity in the pH range from about 6 to 10,which is selected from the group consisting of compounds of the formulaIII

 where n is an integer from 0 to 4 and R¹ is hydrogen or an alkali metalion, and the substituents R² independently of one another are ahydroxyl, alkyl, hydroxyalkyl, heteroalkyl or hydroxyheteroalkyl group.2. An antifreeze concentrate as claimed in claim 1, wherein the reservealkalinity donor is selected from the group consisting of 4,4′- and2,4′-dihydroxydiphenyl sulfone and mixtures thereof.
 3. An antifreezeconcentrate as claimed in claim 1, which comprises at least one reservealkalinity donor of the formula III in a proportion of from about 0.05to 4% by weight, based on the overall weight of the concentrate.
 4. Anantifreeze concentrate as claimed in claim 1, wherein the corrosioninhibitor is at least one selected from the group consisting oftriazoles, thiazoles, and mono- and polybasic, aliphatic and aromaticcarboxylic acids and salts of said acids.
 5. An antifreeze concentrateas claimed in claim 4, which comprises at least one corrosion inhibitorin a proportion of from about 0.1 to 20% by weight, based on the overallweight of the concentrate.
 6. An antifreeze concentrate as claimed inclaim 1, wherein the alcohol which lowers the freezing point is selectedfrom the group consisting of alkylene glycols and alkylene glycolethers, and mixtures thereof.
 7. An antifreeze concentrate as claimed inclaim 1, which is essentially free from borate and silicate.
 8. Acoolant composition comprising an antifreeze concentrate as claimed inclaim
 1. 9. A process for increasing the reserve alkalinity of a coolantcomposition comprising alkylene glycols, alkylene glycol ethers, ormixtures thereof, said process comprising adding to said coolantcomposition, a reserve alkalinity donor compound which is selected fromthe group consisting of compounds of the formula III

where n is an integer from 0 to 4 and R¹ is hydrogen or an alkali metalion, and the substituents R² independently of one another are ahydroxyl, alkyl, hydroxyalkyl, heteroalkyl or hydroxyheteroalkyl group.10. The process of claim 9, wherein the coolant composition isessentially free from borate and silicate.